chemistry revision GCSE

how to revise for 21st century science chemistry GSCE C4,

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periodic table

Dobereiner organised the elements into triads, where every 3 elementsd had a similar chemical property. Eg. Lithium sodium and potassium, where sodium is the average of Li and K. 

On the other hand, Newland made a law of octaves where every 7 elements were in order of relative atomic mass...however, 

  • his groups contained elements that didnt have similar properties like titanium and carbon
  • he mixed up metals and non-metals
  • he didnt leave gaps

 Soon a russian scientist called Dimitri Mendeleev aranged the elements in order of relative atomic mass but left gaps!(for undescovered elements)

{there wasnt much evedidence at that time to show that elements fitted together in that way...  but the undiscovedered elements fitted into the periodic table perfectly}

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The Atom

Atoms make up everything in our lives.

Its is made up of protons(+), neutrons(no charge) and electrons(-).

Nucleus

The nucleus is in the centre of the atoms and has the most mass in the atom. It is tiny compared to the rest of the atom. It holds a positive charge because of the protons.

Electrons-

Electrons are small but cover a lot of space they have virtually no mass. They are arranged in 'shells' and are around the outside of the nucleus.--> if an atom gains or looses an electron it becomes an ion.

Each element has a different number of protons, (the number of protons=the number of electrons) Each atom can hold 2 electrons on its 1st shell, 8 on its 2nd shell and 8 on its 3rd shell. 

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How to understand a periodic table

The periodic table is in order of Atomic number. This is the number to the bottom left of the Symbol. The top left numebr is the RAM or Relative Atomic M*** which represents the total number of protons and neutons. This number can be used to calculate the RFM or Relative Formula m*** of a compound.The atomic number reprents the number of Protons and electrons, You can then work out the electron configuration. To find out the number of neutons in a atom you take the atomic number  away from the RAM.

The verticle Columns in the table represent the different groups usually represented in Roman numerals. These are labeled from either 0-7 or 1-8. These also show how many electrons are in the outer shell of each element.

The rows are called periods. Each new period makes a new outer shell.

As you go from left to right, the elements get less metalic, meaning the elements on the left are metals and the ones on the right are non-metals. The elements in the middle are transition metals.   

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Ionic bonding

Atoms that have gained or lost an electron become a ion and have a charge.Ionic bonding is the transfering of electrons. They are made up of metals and non-metals.

Groups 1,2+3 in the periodic table will have a +positive charge as they will lose a negative electron making them positive(+1,+2 and +3) to make a full outer shell and become stableAt the other end of the Periodic table groups 7,6+5 will gain a negative electron to become stable and make a full outer shell making them have a -negative charge.(-1,-2 and -3)

Atoms with an equal and opposite charge will attract. For example the Alkali metals (group 1) are attracted to the Halogens (group 7)-making an ionic compound.

When solid, Ionic compounds form a lattice ridgid structure like Na+Cl- which makes our table salt and forms only a single crystal. They need to be molten or a liquid to conduct electricity as the ions are able to move.

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Group 1- Alkali metals

These elements need to loose one electron from its outer shell therefor giving thema positive charge.

  • They tarnish in air so have to be stored in oil(to prevent oxidisation) as react with the oxygen and water in the air
  • You must wear gloves when handeling as will react with the sweat on your hands
  • Spontaneously combust 
  • When put in water, they will fizz and produce hydrogen gas [which makes and alkaline solution] and a hydroxide of the metal is formed

2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g)

As you go down the column:Increase in reactivity,melting and boiling point, decreases, density increases and there is a decrease in hardness

When reacted with chlorine, they will produce a salt.

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Group 7-Halogens

All the elements in the halogen group are diatomic (travel round in pairs of atoms)As they are poisonous, they have to be dealt with in a fume cupboard.

As you go down:colour gets darker....state turns from gas to solid...reactivity decreases...melting and boiling point increases...

Fluorine is a very reactive poisonous yellow gas.

Chlorine is a fairly reactive dense and poisonous green gas.

Bromine is a volatile red/brown liquid and an orange gas.

Iodine is a grey solid with a purple gas.

They react with group 1 to create a salt. They also do displacement reactions [where a  more reactive halogen replaces a less reactive halogen from a compound.]

Electrostatic forces between the nucleus and the outer shells of electrons become weaker with the more shells you add so electrons are lost more easily.

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line spectrums

a line spectra is used to identify different positive ions

When heated, some elements emit particular colours:

  • Lithium has a red flame
  • copper has a green flame
  • sodium has a yellow flame 
  • Potassium has a lilac flame

Different elements emit different wavelengths of light, due to the different electron arrangements.Each element has a different colour and a different amount of lines and place of lines on the spectrum.

A technique to produce line spectrums is called a spectroscopy. You can use this to identify different compounds like in forensics.


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Salts

To know what different salts you will make, you will need to know 3 equations:

  • metal + acid ---> salt + hydrogen
  • metal carbonate + acid ---> salt+ water + carbon dioxide
  • metal oxide/hydroxide + acid ---> salt + water

also, to know what salt you make, you need to know what different acids make:

  • nitiric acid makes a ....nitrate
  • sulphuric acid makes a ....sulphate
  • and hydrochloric acid makes a ....chloride

e.g.sodium + hydrochloric acid ---> sodium chloride + hydrogen

Nitric acid + lithium Carbonate ---> lithium nitrate + water + carbon dioxide

Magnesium hydroxide + hydrochloric acid ---> magnesium chloride + water

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